1. Field of the Invention
The present invention relates to a composition comprising a pharmaceutically active agent, such as, but not limited to, an immunogenic agent (e.g., a vaccine), and a bioadhesive delivery system, that allows the oral administration and delivery of the pharmaceutically active agent essentially unaltered to the intestinal mucosa.
2. Background of Related Art
Orally delivered pharmaceutically active agents present a significant problem in transiting an animal's stomach, an organ whose contents represent a harsh digestive environment consisting of low pH and enzymes specifically designed to denature proteins. As a consequence, orally delivered bacterin or subunit vaccines have not been proven to be efficacious since the antigens are generally modified by the stomach prior to presentation to the immuno-responsive cells of the gut mucosa. A number of approaches have been tested to provide an oral delivery vehicle that would transit the stomach but most have been unsuccessful at the commercial scale. One approach involves the transient changing of the stomach pH, neutralizing gastric enzymes and stimulating the mucosal immune response.
In 2003 about 200 million fish were vaccinated in Chile, primarily for Yersiniosis, Salmonid Rickettsial Septicaemia (SRS), and Infectious Pancreatic Necrosis (IPN) (Bravo, 2007). Of the more than 20 vaccines for aquacultured fish brought to the Chilean market from 1999-2003, none were orally delivered vaccines.
SRS is a pathology of salmonid fish caused by the intracellular bacterium Piscrickettsia salmonis and is a major infectious disease in the Chilean salmon industry with annual losses exceeding 20%. Unlike other bacterial diseases, the anti-SRS vaccination is not as effective in preventing the disease or in reducing the need for post-infection medication. This is because of a gradual diminishing of the SRS immunogenicity in the vaccinated fish. Boostering the antibody titer in the blood by vaccinating at a later stage should allow the continued protection of the animals throughout the entire commercial growing period. However, it is extremely difficult and economically impractical to provide parenteral vaccine boosters to large animals in the grow-out net pens.
Almost all existing vaccines are delivered to aquatic animals by injection, which is traumatic, inconvenient, time consuming, expensive, has a number of side effects, and may fail to induce an appropriate immunogenic response in mucosal tissues. Thus, a method and system for delivery that avoids these disadvantages would be of great value.
Perhaps the most well known antigen delivery systems are those derived from the linear polymeric esters of lactic acid and glycolic acid (i.e., poly DL-lactide-co-glycolide, PLGA, reviewed by Wu (Wu, 2004). In such systems, immunogenic subunit vaccine components have been captured in poly-acrylate and poly-glycolide/lactide beads or liposome-like vesicles through processes utilizing volatile organic solvents such as dichloromethane or chloroform. The solvents are used to form emulsions of polymer solutions or dried lipid films. Encapsulation of antigens into PLGA microcapsules affords a number of advantages including rapid degradation by hydrolysis and subsequent penetration of the Peyer's Patches (concentrated sites of lymphocytic tissue in the intestinal mucosa of higher vertebrates but not in fish). A major disadvantage of PLGA microcapsules is the requisite use of organic solvents. Contact with organic solvents can inactivate or reduce the efficacy of the vaccine by altering the immunogenicity of surface proteins critical to induction of humoral or cellular immune responses. Additionally, poly-acrylate and poly-glycolide/lactide processes typically result in microbeads with extremely low immunogen or antigen capture efficiency.
Polymer microspheres and lamellar particles (e.g., liposomes) have been employed for the improved parenteral and mucosal administration of antigens. Because vaccines themselves may not be efficiently recognized and taken up by mucosal lymphocytes, they typically need to be co-administered with penetration enhancers or adjuvants. Different classes of polymer mixtures are known for potential use as Mucoadhesives(Malik et al., 2007). These include synthetic polymers such as poly (acrylic acid) (PAA), hydroxypropyl methylcellulose and poly(methylacrylate) derivatives, as well as naturally occurring polymers such as hyaluronic acid and chitosan.
Chitosan and various chitosan derivatives have been used for a variety of applications as a biomaterial for tissue engineering, wound healing, and as an excipient for drug delivery (Chopra et al., 2006; Dang and Leong, 2006). Chitosan has occasionally been tested as an adjuvant for mucosal application (Kim et al., 2007), but it is typically applied directly to a mucosal surface such as intranasal application in order to obtain IgA response in the nasopharyngeal mucosa of terrestrial animals (Kang et al., 2007). However, the use of chitosan and various chitosan derivatives in vaccine delivery remains very limited due to poor physicochemical characteristics such as a high transition temperature and interfacial free energy, resulting in a suboptimal interaction with mucosal surfaces and loose interpenetration and interdiffusion of the polymer. This problem is further compounded when used for poikilothermic lower vertebrates like salmonid fish. Chitosan also has the additional disadvantage of a low mechanical strength and solubility.
Thus, there remains a need for effective systems and processes for microencapsulation of immunogenic substances with polymers having superior adhesive and cohesive properties.